It is common to administer a diluted secondary medication intravenously to a patient followed by a primary medication. A commonly used system is referred to as a piggy-back setup. Generally, a catheter is located in a patient's vein and connected by a delivery tube to a "Y" connector. One branch of the connector is coupled through a one-way valve to a primary reservoir which contains the primary medication such as saline. The other branch of the connector is coupled with a secondary reservoir into which the secondary medication is placed. The secondary medication can be a drug diluted in a liquid vehicle such as saline.
For proper operation, the secondary reservoir is hung at a higher elevation than the primary reservoir. The secondary medication then has a greater pressure head than the primary medication. This pressure difference keeps the valve closed so that only diluted secondary medication flows to the patient. When the secondary reservoir is exhausted, its pressure head decreases to zero and the valve opens. The primary medication then flows to the patient. For examples of piggy-back setups, see U.S. Pat. Nos. 3,886,937 to Bobo et al. and 4,372,306 to Genese et al.
Unfortunately, such systems require two reservoirs, one for the primary medication and one for the secondary medication. The amount of secondary medication dilution is restricted by the size of the secondary reservoir. The liquid level in the secondary reservoir must also be maintained at a greater height than the liquid level in the primary reservoir. Such systems further require two separate sets of tubing, a connector, a valve, and related flow regulation apparatus. Such an extensive amount of equipment not only complicates the use of the system, but also adds to its cost.
It is also possible for the valve to stick open causing a mixture of the secondary medication with the primary medication. This changes the rate and dilution of medication from that which has been prescribed. The flow regulation apparatus in such systems is usually associated with the delivery tube below the "Y" connector. This causes difficulties in selecting and maintaining the administration rate as the relative reservoir pressure heads vary.
To avoid some of these problems and provide for a greater range of medication dilution, a second type of system has been developed. This system uses a burette connected by a first tube to a primary reservoir. The desired secondary medication is injected into the burette and the appropriate amount of primary medication is added from the primary reservoir. The air displaced by the primary medication is vented out of the burette through a filter material. The flow through the first tube is stopped and the diluted medication is administered through a second tube to the patient as air enters the burette through the filter material. If desired, more medication can be diluted or the flow of primary medication through the first tube restarted to flow on to the patient.
While this system avoids some of the problems of the previously discussed system, it has its own shortcomings. Among these are the possibility of contamination entering through the filter material as the medication solution is delivered. It is not possible to automatically sequentially deliver the primary mediction after the delivery of the secondary medication has been completed. An operator must return to adjust the flow regulator after the delivery of the secondary medication has been completed.
Accordingly, it would be desirable to avoid the difficulties encountered with present systems and provide a device which provides for uniform and adjustable dilutions of medication. The device should also be able to automatically administer the primary medication alone after the desired amount of secondary medication has been administered. It would be further desirable if such a device were easy to set up, inexpensive to manufacture, and relatively simple to use. The present invention meets these desires.